27 Engineering Geology (Posters)

LONGITUDINAL SURVEY OF A LOBATE ROCK GLACIER USING GROUND PENETRATING RADAR (GPR), YANKEE BOY BASIN, CO, USA

Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Rock glaciers are lobate or tongue-shaped bodies composed of mixtures of poorly sorted, angular, blocky rock debris and ice. They are located in alpine cirques and valleys in many mountain ranges throughout the world. Engineering and climatological considerations make it important to understand the manner in which deformation in rock glaciers occurs. The geomorphology of rock glaciers and other lobate landforms with surficial ridges and furrows perpendicular to flow direction is well documented, however, our current knowledge of their movement mechanics and flow behavior is based on limited data and estimations derived from glacial creep models such as Glen’s Flow Law. Thus, the basic parameter for characterizing observable characteristics of rock glaciers seems to involve the nature and amount of ice within a rock glacier.

A 460 m longitudinal GPR profile (trending SW-NE) was collected on the longest continuous flow lobe of the rock glacier using 25 MHz antennae and a 1000V transmitter. Reflections recorded in excess of 650 ns (approximately 40 m) appear to include the contact between rock glacier and valley floor in the head area of the rock glacier. The profile indicates well-defined, continuous and semi-continuous layers as well as large boulder-sized rocks below the surface. A distinct reflection at mid-profile is interpreted to represent the discernable contact of one flow lobe overriding another. Amplitude and frequency of folds in the layers also increases in association with pronounced ridges and furrows, suggesting that material comprising the rock glacier is compressed toward the toe with decreasing valley gradient. The folds indicate that waveform energy is transmitted deep inside the landform, and that ridge-furrow topography is a surface manifestation of this process. Based on this evidence, a kinematic wave model may provide a useful means to describe propagation of matter and energy in the rock glacier and to resolve observed discrepancies between talus supply rate and rock glacier rate of movement.